In a semiconductor manufacturing process, washing of electronic parts, and washing of medical apparatuses, high-purity pure water that does not contain impurities is used. In general, this type of pure water is produced by processing raw water, such as groundwater and tap water, with a reverse osmosis membrane (hereinafter, also an “RO membrane”).
At the time of producing pure water with an RO membrane, there occurs a phenomenon that a hardness component contained in raw water is precipitated as a scale on a surface of the RO membrane. Further, there also occurs a phenomenon of what is called fouling that a suspended substance (colloidal iron in an insolubilized state, for example) contained in the raw water is deposited on a surface of the RO membrane or within pores. When precipitation of a scale and fouling occur on the RO membrane, permeability (water permeability coefficient) of the RO membrane decreases. As a result, a flow rate of permeate water decreases.
To suppress precipitation of a calcium carbonate scale, a system that supplies soft water to the RO membrane is being proposed. This system softens raw water (hard water), by a water softening device that uses a cation exchange resin as a preprocessing of the RO device.
On the other hand, in an RO membrane in a state that precipitation of a scale and fouling are not generated, a water permeability coefficient changes based on a temperature of water that is supplied (hereinafter, “supply water”). The water permeability coefficient of the RO membrane becomes larger when a temperature of the supply water is higher. Therefore, even when the supply water is supplied to the RO membrane at a constant pressure, a flow rate of permeate water increases when a temperature becomes higher. When the flow rate of the permeate water increases, concentration of soft water proceeds at a primary side of the RO membrane.
In general, a water softening device cannot remove a silica component and a suspended substance that are contained in raw water. Therefore, when concentration of soft water proceeds at a primary side of an RO membrane, precipitation of a silica scale and fouling easily occur. That is, when soft water is added to the RO membrane, the occurrence of precipitation of a silica scale and fouling cannot be suppressed depending on a temperature, even when precipitation of a calcium carbonate scale can be suppressed.
To keep constant a flow rate of permeate water in the RO membrane regardless of a temperature of supply water, a system that controls a flow rate feedback is being proposed. According to this flow rate feedback control, an operation frequency of a pressure pump that delivers supply water to the RO membrane is controlled by an inverter equipment such that a flow rate of permeate water that is produced by the RO membrane becomes a target value.
However, according to the conventional water softening device, it is difficult to produce high-purity soft water from which a hardness leakage level is sufficiently decreased, by using hard water of poor water quality. Further, according to the conventional water softening device, it is difficult to secure a practical water collection volume, even when high-purity soft water can be produced from hard water of poor water quality. Therefore, it is difficult to stably suppress precipitation of a calcium carbonate scale in the RO membrane.